We are applying an in vitro colony forming assay system to the study of primary urothelial explants obtained transurethrally by bladder barbotage and/or tumor biopsy. Samples are mechanically disassociated into a single cell suspension and plated in bi-layered agar support system. A small fraction of cells plated form colonies in vitro within 10-21 days. Urothelial cells from "control" patients obtained by bladder barbotage form colonies in 25% of the cases studied while 80-90% of samples from bladder cancer patients form colonies. Such clonal growth greatly facilitates karyotypic studies. If tumor cells are exposed to anti-cancer drugs in vitro prior to plating, a reduction in colony forming ability as compared to untreated control cells from the same patient provides a measure of "drug sensitivity". Studies to date in over 20 bladder cancer patients demonstrate a heterogeneous in vitro response to cytotoxic agents. Such studies in other tumors demonstrate good correlation between in vitro/in vivo response to anticancer drugs and similar clinical trials correlating in vitro and in vivo responses for bladder cancer are ongoing at our institution for both systemically and intravesically administered agents.